1. Field of the Invention
The present invention relates to a semiconductor laser capable of changing a polarization mode of its output light, a semiconductor laser apparatus and a driving method therefor.
2. Related Background Art
Conventionally, Japanese Patent Laid-Open Application No. 2 (1990)-159781 discloses a three-electrode distributed feedback (DFB) laser with a .lambda./4 phase-shift section whose polarization mode of output light can be varied between the TE mode and the TM mode depending on its stimulated condition. In the DFB semiconductor laser of that reference, there are arranged three electrodes for independently injecting currents into a region with the .lambda./4 phase-shift section and other regions without any .lambda./4 phase-shift sections. The polarization-mode modulation operation of this laser device is performed as follows. The laser is brought into a laser-oscillated condition by appropriately injecting a current through each electrode, and the polarization mode of its output light is changed between the TE mode and the TM mode by slightly varying the current injected into the region with the .lambda./4 phase-shift section under the laser-oscillated condition. Thus, in the above-discussed three-electrode DFB semiconductor laser, the current injected into the region with the .lambda./4 phase-shift section is varied and its equivalent refractive index is accordingly changed such that the oscillation occurs at a wavelength and in the TE mode or the TM mode which satisfy the device's phase matching condition and take a minimum threshold gain.
Further, Japanese Patent Laid-Open Application No. 7 (1995)-162088 discloses a two-electrode DFB laser which has a simple structure. In this DFB laser, its oscillation polarization mode is switched by injecting different currents through the two electrodes and modulating one of the two different currents. In this structure, a difference between Bragg wavelengths for the TE mode and the TM mode in one region is made different from that in another region by injecting different bias currents through the two electrodes and the current injected into one region is varied under this stimulated condition such that its oscillation polarization mode is changed.
Furthermore, Japanese Patent Laid-Open Application No. 8 (1996)-274412 discloses a polarization-mode selective DFB laser in which equivalent refractive indices of respective regions are made structurally different from each other along its cavity direction. In this structure, there are arranged regions having different refractive indices in its cavity direction, and hence differences between propagation constants (i.e, differences between equivalent refractive indices) for the TE mode and the TM mode in the respective regions are made different from each other. Thus, it is possible to switch the oscillation polarization mode more stably than another conventional structure, such as the structure as disclosed in Japanese Patent Laid-Open Application No. 2 (1990)-159781.